Many Signalcrafters customers are in the
business of protecting electric transmission lines from fault
damage. Among the methods used to accomplish this is one wherein
radio signals are coupled directly onto the high voltage lines
themselves. This type of communication is known as
"POWERLINE CARRIER" (PLC). The information communicated
is called "PROTECTIVE RELAYING", where relays at the
ends of the transmission line are instructed to operate,
de-energizing the line whenever conditions on that line exceed
predetermined values.

A LINE TRAP IS

As you can imagine, it takes some unusual
engineering to couple a radio signal onto an antenna carrying
500,000 volts. Nonetheless it is done everyday. One of the
devices employed in Powerline Carrier is the LINE TRAP, sometimes
called a wave trap. A line trap is a parallel resonant circuit
installed on the powerline at the local substation. It is tuned
to resonate at a specific frequency or frequencies. These
frequencies are equivalent to the TxRx frequencies of the local
powerline carrier transceiver. Properly tuned, the line trap
shows its highest magnitude of impedance, ç Z ç , at these carrier
frequencies while permitting the 60 Hz power frequency to pass.

OLD TUNING METHOD IS COSTLY

Tuning these devices in the past has been an
expensive, time consuming task prone to error. The accepted
method to accomplish this task employed a signal generator, a
frequency counter, a VTVM, a frequency selective level meter and
a resistor plus all the interconnecting leads and transitions.
Furthermore, the trap had to be removed from the powerline. What
one did in this method was to place the known value resistor in
series with the trap and measure the voltage across it with the
level meter at a specific frequency. Using Ohms law, a current
value can be obtained. Now, with this data, one could calculate
the ç
Z ç
the trap showed at that frequency. One then would have to repeat
this procedure over and over as required to determine the
traps impedance vs. frequency characteristics over its
bandwidth.

THE MODEL 60 SOLUTION

Signalcrafters Model 60 Impedance Magnitude
Meter eliminates all of that complex data taking/calculating. In
doing so, the time required to perform the task is reduced from
hours to minutes. Furthermore, using the Model 60 for the job
involves far fewer expensive instruments. With the Model 60, one
simply connects the two test leads across the wave traps mounting
ears (the trap neednt be removed from the line). Then one
provides a 10 Vrms signal on a frequency of interest at the
"Sig In" port of the instrument and it instantly reads
the magnitude of the traps impedance displayed in Ohms directly
on the analog front panel meter. If the signal source driving the
Model 60 is capable of a swept output, one can see the traps ç Z ç displayed
dynamically.

WHAT HAPPENS IS

The Model 60 contains an internal current
transformer to convert the traps input voltage to current through
the trap. The Model 60 also measures the voltage across the trap.
An on board analog computer then does all the ratio-taking
mathematics to produce the displayed magnitude of impedance.

If a change in the line traps resonant
frequency is desired, one simply adjusts the signal generators
output to the new frequency, then adjusts the traps inductance or
capacitance to again produce a peak reading on the Model 60.